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1
Ge, Yan, Gavin Gong, and Allan Frei. "Physical Mechanisms Linking the Winter Pacific–North American Teleconnection Pattern to Spring North American Snow Depth." Journal of Climate 22, no. 19 (2009): 5135–48. http://dx.doi.org/10.1175/2009jcli2842.1.
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Abstract The wintertime Pacific–North American (PNA) teleconnection pattern has previously been shown to influence springtime snow conditions over portions of North America. This paper develops a more complete physical understanding of this linkage across the continent, using a recently released long-term, continental-scale gridded North American snow depth dataset and the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis data. An empirical orthogonal function–based filtering process is used to identify and isolate the interannual snow depth variations associated with PNA. Then linear and partial correlations are employed to investigate the physical mechanisms that link winter PNA with spring snow depth. In the positive phase of PNA, the enhanced PNA pressure centers lead to warmer temperatures over northwestern North America and less precipitation at midlatitudes. The temperature and precipitation pathways act independently and in distinct geographical regions, and together they serve to reduce winter snow depth across much of North America. Winter anomalies in the snow depth field then tend to persist into spring. Dynamic mechanisms responsible for the PNA-influenced North American precipitation and temperature anomalies, involving moisture transport and cold air intrusions, are confirmed in this study and also extended to continental snow depth anomalies.
2
Liu, Zhongfang, Yanlin Tang, Zhimin Jian, Christopher J. Poulsen, Jeffrey M. Welker, and Gabriel J. Bowen. "Pacific North American circulation pattern links external forcing and North American hydroclimatic change over the past millennium." Proceedings of the National Academy of Sciences 114, no. 13 (2017): 3340–45. http://dx.doi.org/10.1073/pnas.1618201114.
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Land and sea surface temperatures, precipitation, and storm tracks in North America and the North Pacific are controlled to a large degree by atmospheric variability associated with the Pacific North American (PNA) pattern. The modern instrumental record indicates a trend toward a positive PNA phase in recent decades, which has led to accelerated warming and snowpack decline in northwestern North America. The brevity of the instrumental record, however, limits our understanding of long-term PNA variability and its directional or cyclic patterns. Here we develop a 937-y-long reconstruction of the winter PNA based on a network of annually resolved tree-ring proxy records across North America. The reconstruction is consistent with previous regional records in suggesting that the recent persistent positive PNA pattern is unprecedented over the past millennium, but documents patterns of decadal-scale variability that contrast with previous reconstructions. Our reconstruction shows that PNA has been strongly and consistently correlated with sea surface temperature variation, solar irradiance, and volcanic forcing over the period of record, and played a significant role in translating these forcings into decadal-to-multidecadal hydroclimate variability over North America. Climate model ensembles show limited power to predict multidecadal variation in PNA over the period of our record, raising questions about their potential to project future hydroclimatic change modulated by this circulation pattern.
3
Li, Xiaofan, Zeng-Zhen Hu, Ping Liang, and Jieshun Zhu. "Contrastive Influence of ENSO and PNA on Variability and Predictability of North American Winter Precipitation." Journal of Climate 32, no. 19 (2019): 6271–84. http://dx.doi.org/10.1175/jcli-d-19-0033.1.
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Abstract In this work, the roles of El Niño–Southern Oscillation (ENSO) in the variability and predictability of the Pacific–North American (PNA) pattern and precipitation in North America in winter are examined. It is noted that statistically about 29% of the variance of PNA is linearly linked to ENSO, while the remaining 71% of the variance of PNA might be explained by other processes, including atmospheric internal dynamics and sea surface temperature variations in the North Pacific. The ENSO impact is mainly meridional from the tropics to the mid–high latitudes, while a major fraction of the non-ENSO variability associated with PNA is confined in the zonal direction from the North Pacific to the North American continent. Such interferential connection on PNA as well as on North American climate variability may reflect a competition between local internal dynamical processes (unpredictable fraction) and remote forcing (predictable fraction). Model responses to observed sea surface temperature and model forecasts confirm that the remote forcing is mainly associated with ENSO and it is the major source of predictability of PNA and winter precipitation in North America.
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Hu, Yongyun, Yan Xia, Zhengyu Liu, Yuchen Wang, Zhengyao Lu, and Tao Wang. "Distorted Pacific–North American teleconnection at the Last Glacial Maximum." Climate of the Past 16, no. 1 (2020): 199–209. http://dx.doi.org/10.5194/cp-16-199-2020.
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Abstract. The Pacific–North American (PNA) teleconnection is one of the most important climate modes in the present climate condition, and it enables climate variations in the tropical Pacific to exert a significant influence on North America. Here, we show climate simulations in which the PNA teleconnection was largely distorted or broken at the Last Glacial Maximum (LGM). The distorted PNA is caused by a split in the westerly jet stream, which is ultimately forced by the large, thick Laurentide ice sheet that was present at the LGM. Changes in the jet stream greatly alter the extratropical waveguide, distorting wave propagation from the North Pacific to North America. The distorted PNA suggests that climate variability in the tropical Pacific, notably El Niño–Southern Oscillation (ENSO), would have little direct impact on North American climate at the LGM.
5
Ge, Yan, and Gavin Gong. "North American Snow Depth and Climate Teleconnection Patterns." Journal of Climate 22, no. 2 (2009): 217–33. http://dx.doi.org/10.1175/2008jcli2124.1.
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Abstract Snow–atmosphere relationships have been studied for nearly half a century, but the primary focus has been on snow extent variability, largely because of the relative scarcity of snow depth data. A recently released North American snow depth dataset, with extensive spatial coverage and multidecadal temporal duration, provides a new opportunity to compare snow depth–climate relationships with snow extent–climate relationships over North America. Robust concurrent lead and lag correlations are observed between snow depth and two major climate modes, the Pacific decadal oscillation (PDO) and the Pacific–North America (PNA) pattern, across North America and throughout the snow season. In contrast, snow extent exhibits a less coherent relationship with PDO and PNA except in late spring, which can be interpreted as a residual of the snow depth–climate mode relationship. A regional signature for the snow depth–PDO/PNA relationship is also identified, centered over interior central-western North America. Smaller scales mask the regional effect of PDO and PNA because of local snow depth variability, while larger continental scales exceed the regional domain of the climate mode teleconnections. Overall these results suggest that North American snow depth variability may have greater climatic causes and consequences than snow extent. Physical mechanisms that may be responsible for the observed snow depth–climate teleconnection patterns such as the surface energy balance, moisture transport, and atmospheric flow regimes are briefly discussed.
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Yu, B., H. Lin, V. V. Kharin, and X. L. Wang. "Interannual Variability of North American Winter Temperature Extremes and Its Associated Circulation Anomalies in Observations and CMIP5 Simulations." Journal of Climate 33, no. 3 (2020): 847–65. http://dx.doi.org/10.1175/jcli-d-19-0404.1.
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AbstractThe interannual variability of wintertime North American surface temperature extremes and its generation and maintenance are analyzed in this study. The leading mode of the temperature extreme anomalies, revealed by empirical orthogonal function (EOF) analyses of December–February mean temperature extreme indices over North America, is characterized by an anomalous center of action over western-central Canada. In association with the leading mode of temperature extreme variability, the large-scale atmospheric circulation features an anomalous Pacific–North American (PNA)-like pattern from the preceding fall to winter, which has important implications for seasonal prediction of North American temperature extremes. A positive PNA pattern leads to more warm and fewer cold extremes over western-central Canada. The anomalous circulation over the PNA sector drives thermal advection that contributes to temperature anomalies over North America, as well as a Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomaly pattern in the midlatitude North Pacific. The PNA-like circulation anomaly tends to be supported by SST warming in the tropical central-eastern Pacific and a positive synoptic-scale eddy vorticity forcing feedback on the large-scale circulation over the PNA sector. The leading extreme mode–associated atmospheric circulation patterns obtained from the observational and reanalysis data, together with the anomalous SST and synoptic eddy activities, are reasonably well simulated in most CMIP5 models and in the multimodel mean. For most models considered, the simulated patterns of atmospheric circulation, SST, and synoptic eddy activities have lower spatial variances than the corresponding observational and reanalysis patterns over the PNA sector, especially over the North Pacific.
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Ghatak, Debjani, Gavin Gong, and Allan Frei. "North American Temperature, Snowfall, and Snow-Depth Response to Winter Climate Modes." Journal of Climate 23, no. 9 (2010): 2320–32. http://dx.doi.org/10.1175/2009jcli3050.1.
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Abstract The snowpack is an important seasonal surface water storage reservoir that affects the availability of water resources during the spring and summer seasons in mid–high latitudes. Not surprisingly, interannual variations in snow cover extent and snow water equivalent have been extensively studied in arid regions such as western North America. This study broadens the focus by examining snow depth as an alternative snowpack metric, and considers its variability over different parts of North America. The authors use singular value decomposition (SVD) in conjunction with linear and partial correlation to show that regional snow-depth variations can be largely explained by the winter North Atlantic Oscillation (NAO) and the Pacific–North American (PNA) modes of atmospheric variability through distinct mechanistic pathways involving regional winter circulation patterns and hydrologic fluxes. The high index phase of the NAO generates positive winter air temperature anomalies over eastern parts of North America, causing thinning of the winter snowpack via snowmelt. Meanwhile, the high index phase of the PNA generates negative winter snowfall anomalies across midlatitudinal areas of North America, which also serve to thin the snowpack. Positive PNA anomalies have also been shown to increase temperatures and decrease snow depths over western North America. The PNA influence extends across the continent, whereas the NAO influence is limited to eastern North America. The winter snow-depth variations associated with all of these pathways exhibit seasonal persistence, which ultimately yield regional-scale spring snow-depth anomalies throughout much of North America.
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Schreck, Carl J., Jason M. Cordeira, and David Margolin. "Which MJO Events Affect North American Temperatures?" Monthly Weather Review 141, no. 11 (2013): 3840–50. http://dx.doi.org/10.1175/mwr-d-13-00118.1.
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Abstract Tropical convection from the Madden–Julian oscillation (MJO) excites and amplifies extratropical Rossby waves around the globe. This forcing is reflected in teleconnection patterns like the Pacific–North American (PNA) pattern, and it can ultimately result in temperature anomalies over North America. Previous studies have not explored whether the extratropical response might vary from one MJO event to another. This study proposes a new index, the multivariate PNA (MVP), to identify variations in the extratropical waveguide over the North Pacific and North America that might affect the response to the MJO. The MVP is the first combined EOF of 20–100-day OLR, 850-hPa streamfunction, and 200-hPa streamfunction over the North Pacific and North America. The North American temperature patterns that follow each phase of the MJO change with the sign of the MVP. For example, real-time multivariate MJO (RMM) phase 5 usually leads to warm anomalies over eastern North America. This relationship was only found when the MVP was negative, and it was not associated with El Niño or La Niña. RMM phase 8, on the other hand, usually leads to cold anomalies. Those anomalies only occur if the MVP is positive, which happens somewhat more frequently during La Niña years. Composite analyses based on combinations of the MJO and the MVP show that variability in the Pacific jet and its associated wave breaking play a key role in determining whether and how the MJO affects North American temperatures.
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Luo, Dehai, Yao Ge, Wenqi Zhang, and Aiguo Dai. "A Unified Nonlinear Multiscale Interaction Model of Pacific–North American Teleconnection Patterns." Journal of the Atmospheric Sciences 77, no. 4 (2020): 1387–414. http://dx.doi.org/10.1175/jas-d-19-0312.1.
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Abstract In this paper, reanalysis data are first analyzed to reveal that the individual negative (positive)-phase Pacific–North American pattern (PNA) or PNA− (PNA+) has a lifetime of 10–20 days, is characterized by strong (weak) westerly jet stream meanders, and exhibits clear wave train structures, whereas the PNA− with rapid retrogression tends to have longer lifetime and larger amplitude than the PNA+ with slow retrogression. In contrast, the wave train structure of the North Atlantic Oscillation (NAO) is less distinct, and the positive (negative)-phase NAO shows eastward (westward) movement around a higher latitude than the PNA. Moreover, it is found that the PNA wave train occurs under a larger background meridional potential vorticity gradient (PVy) over the North Pacific than that over the North Atlantic for the NAO. A unified nonlinear multiscale interaction (UNMI) model is then developed to explain why the PNA as a nonlinear wave packet has such characteristics and its large difference from the NAO. The model results reveal that the larger background PVy for the PNA (due to its location at lower latitudes) leads to its larger energy dispersion and weaker nonlinearity than the NAO, thus explaining why the PNA (NAO) is largely a linear (nonlinear) process with a strong (weak) wave train structure, though it is regarded as a nonlinear initial-value problem. The smaller PVy for the PNA− than for the PNA+ leads to lower energy dispersion and stronger nonlinearity for PNA−, which allows it to maintain larger amplitude and have a longer lifetime than the PNA+. Thus, the difference in the background PVy is responsible for the asymmetry between the two phases of PNA and the difference between the PNA and NAO.
10
Yu, Bin, Hai Lin, and Nicholas Soulard. "A Comparison of North American Surface Temperature and Temperature Extreme Anomalies in Association with Various Atmospheric Teleconnection Patterns." Atmosphere 10, no. 4 (2019): 172. http://dx.doi.org/10.3390/atmos10040172.
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The atmospheric teleconnection pattern reflects large-scale variations in the atmospheric wave and jet stream, and has pronounced impacts on climate mean and extremes over various regions. This study compares those patterns that have significant circulation anomalies over the North Pacific–North American–North Atlantic sector, which directly influence surface temperature and temperature extremes over North America. We analyze the pattern associated anomalies of surface temperature and warm and cold extremes over North America, during the northern winter and summer seasons. In particular, we assess the robustness of the regional temperature and temperature extreme anomaly patterns by evaluating the field significance of these anomalies over North America, and quantify the percentages of North American temperature and temperature extreme variances explained by these patterns. The surface temperature anomalies in association with the Pacific–North American pattern (PNA), Tropical–Northern Hemisphere pattern (TNH), North Pacific pattern (NP), North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Western Pacific pattern (WP), circumglobal teleconnection (CGT), and Asian–Bering–North American (ABNA) patterns are similar to those reported in previous studies based on various datasets, indicating the robustness of the results. During winter, the temperature anomaly patterns considered are field significant at the 5% level over North America, except the WP-related one. These pattern associated anomalies explained about 5–15% of the total interannual temperature variance over North America, with relatively high percentages for the ABNA and PNA patterns, and low for the WP pattern. The pattern associated warm and cold extreme anomalies resemble the corresponding surface mean temperature anomaly patterns, with differences mainly in magnitude of the anomalies. Most of the anomalous extreme patterns are field significant at the 5% level, except the WP-related patterns. These extreme anomalies explain about 5–20% of the total interannual variance over North America. During summer, the pattern-related circulation and surface temperature anomalies are weaker than those in winter. Nevertheless, all of the pattern associated temperature anomalies are of field significance at the 5% level over North America, except the PNA-related one, and explain about 5–10% of the interannual variance. In addition, the temperature extreme anomalies, in association with the circulation patterns, are comparable in summer and winter. Over North America, the NP-, WP-, ABNA-, and CGT-associated anomalies of warm extremes are field significant at the 5% level and explain about 5–15% of the interannual variance. Most of the pattern associated cold extreme anomalies are field significant at the 5% level, except the PNA and NAO related anomalies, and also explain about 5–15% of the interannual variance over North America.
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Zhou, Zhen-Qiang, Shang-Ping Xie, Xiao-Tong Zheng, Qinyu Liu, and Hai Wang. "Global Warming–Induced Changes in El Niño Teleconnections over the North Pacific and North America." Journal of Climate 27, no. 24 (2014): 9050–64. http://dx.doi.org/10.1175/jcli-d-14-00254.1.
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Abstract El Niño–Southern Oscillation (ENSO) induces climate anomalies around the globe. Atmospheric general circulation model simulations are used to investigate how ENSO-induced teleconnection patterns during boreal winter might change in response to global warming in the Pacific–North American sector. As models disagree on changes in the amplitude and spatial pattern of ENSO in response to global warming, for simplicity the same sea surface temperature (SST) pattern of ENSO is prescribed before and after the climate warming. In a warmer climate, precipitation anomalies intensify and move eastward over the equatorial Pacific during El Niño because the enhanced mean SST warming reduces the barrier to deep convection in the eastern basin. Associated with the eastward shift of tropical convective anomalies, the ENSO-forced Pacific–North American (PNA) teleconnection pattern moves eastward and intensifies under the climate warming. By contrast, the PNA mode of atmospheric internal variability remains largely unchanged in pattern, suggesting the importance of tropical convection in shifting atmospheric teleconnections. As the ENSO-induced PNA pattern shifts eastward, rainfall anomalies are expected to intensify on the west coast of North America, and the El Niño–induced surface warming to expand eastward and occupy all of northern North America. The spatial pattern of the mean SST warming affects changes in ENSO teleconnections. The teleconnection changes are larger with patterned mean warming than in an idealized case where the spatially uniform warming is prescribed in the mean state. The results herein suggest that the eastward-shifted PNA pattern is a robust change to be expected in the future, independent of the uncertainty in changes of ENSO itself.
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Zanchettin, D., O. Bothe, F. Lehner, P. Ortega, C. C. Raible, and D. Swingedouw. "Reconciling reconstructed and simulated features of the winter Pacific–North-American pattern in the early 19th century." Climate of the Past Discussions 10, no. 6 (2014): 4425–68. http://dx.doi.org/10.5194/cpd-10-4425-2014.
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Abstract. Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree-rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early-19th-century positive PNA anomaly emerges as a potentially reliable feature, although it is subject to a number of sources of uncertainty and potential deficiencies. The pseudo-reconstructions demonstrate that the early-19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early-19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, hence unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early-19th-century external forcing and background climate conditions.
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Zanchettin, D., O. Bothe, F. Lehner, P. Ortega, C. C. Raible, and D. Swingedouw. "Reconciling reconstructed and simulated features of the winter Pacific/North American pattern in the early 19th century." Climate of the Past 11, no. 6 (2015): 939–58. http://dx.doi.org/10.5194/cp-11-939-2015.
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Abstract. Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows for assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early 19th-century positive PNA anomaly emerges as a potentially reliable feature, although the pseudo-reconstructions are subject to a number of sources of uncertainty and deficiencies highlighted especially at multidecadal and centennial timescales. The pseudo-reconstructions demonstrate that the early 19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early 19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, which is unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early 19th-century external forcing and background climate state.
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Yu, B., A. Shabbar, and F. W. Zwiers. "The Enhanced PNA-Like Climate Response to Pacific Interannual and Decadal Variability." Journal of Climate 20, no. 21 (2007): 5285–300. http://dx.doi.org/10.1175/2007jcli1480.1.
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Abstract This study provides further evidence of the impacts of tropical Pacific interannual [El Niño–Southern Oscillation (ENSO)] and Northern Pacific decadal–interdecadal [North Pacific index (NPI)] variability on the Pacific–North American (PNA) sector. Both the tropospheric circulation and the North American temperature suggest an enhanced PNA-like climate response and impacts on North America when ENSO and NPI variability are out of phase. In association with this variability, large stationary wave activity fluxes appear in the mid- to high latitudes originating from the North Pacific and flowing downstream toward North America. Atmospheric heating anomalies associated with ENSO variability are confined to the Tropics, and generally have the same sign throughout the troposphere with maximum anomalies at 400 hPa. The heating anomalies that correspond to the NPI variability exhibit a center over the midlatitude North Pacific in which the heating changes sign with height, along with tropical anomalies of comparable magnitudes. Atmospheric heating anomalies of the same sign appear in both the tropical Pacific and the North Pacific with the out-of-phase combination of ENSO and NPI. Both sources of variability provide energy transports toward North America and tend to favor the occurrence of stationary wave anomalies.
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Drouard, Marie, Gwendal Rivière, and Philippe Arbogast. "The Link between the North Pacific Climate Variability and the North Atlantic Oscillation via Downstream Propagation of Synoptic Waves." Journal of Climate 28, no. 10 (2015): 3957–76. http://dx.doi.org/10.1175/jcli-d-14-00552.1.
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Abstract The North Atlantic Oscillation (NAO) response to the northeast Pacific climate variability is examined using the ERA-40 dataset. The main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study: a low-frequency planetary-scale ridge (trough) anomaly located in the eastern Pacific–North American sector induces more equatorward (poleward) propagation of synoptic-scale wave packets on its downstream side, which favors the occurrence of anticyclonic (cyclonic) wave breakings in the Atlantic sector and the positive (negative) NAO phase. The mechanism first provides an interpretation of the canonical impact of the El Niño–Southern Oscillation on the NAO in late winter. The wintertime relationship between the Pacific–North American oscillation (PNA) and the NAO is also investigated. For out-of-phase fluctuations between the PNA and NAO indices (i.e., the most recurrent situation in late winter), the eastern Pacific PNA ridge (trough) anomaly modifies the direction of downstream wave propagation, triggering more anticyclonic (cyclonic) wave breakings over the North Atlantic. For in-phase fluctuations, the effect of the eastern Pacific PNA anomalies is cancelled out by the North American PNA anomalies. The latter anomalies being deeper and more centered in the latitudinal band of downstream wave propagation, they are able to reverse the direction of wave propagation just before waves enter the Atlantic domain. The contrasting relationship between the PNA and NAO is similar to what occurs for the two leading hemispheric EOFs of geopotential height: the northern annular mode (NAM) and the cold ocean–warm land (COWL) pattern. The proposed mechanism provides a physical meaning for the NAM and COWL patterns.
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Grise, Kevin M., Seok-Woo Son, and John R. Gyakum. "Intraseasonal and Interannual Variability in North American Storm Tracks and Its Relationship to Equatorial Pacific Variability." Monthly Weather Review 141, no. 10 (2013): 3610–25. http://dx.doi.org/10.1175/mwr-d-12-00322.1.
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Abstract Extratropical cyclones play a principal role in wintertime precipitation and severe weather over North America. On average, the greatest number of cyclones track 1) from the lee of the Rocky Mountains eastward across the Great Lakes and 2) over the Gulf Stream along the eastern coastline of North America. However, the cyclone tracks are highly variable within individual winters and between winter seasons. In this study, the authors apply a Lagrangian tracking algorithm to examine variability in extratropical cyclone tracks over North America during winter. A series of methodological criteria is used to isolate cyclone development and decay regions and to account for the elevated topography over western North America. The results confirm the signatures of four climate phenomena in the intraseasonal and interannual variability in North American cyclone tracks: the North Atlantic Oscillation (NAO), the El Niño–Southern Oscillation (ENSO), the Pacific–North American pattern (PNA), and the Madden–Julian oscillation (MJO). Similar signatures are found using Eulerian bandpass-filtered eddy variances. Variability in the number of extratropical cyclones at most locations in North America is linked to fluctuations in Rossby wave trains extending from the central tropical Pacific Ocean. Only over the far northeastern United States and northeastern Canada is cyclone variability strongly linked to the NAO. The results suggest that Pacific sector variability (ENSO, PNA, and MJO) is a key contributor to intraseasonal and interannual variability in the frequency of extratropical cyclones at most locations across North America.
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Harding, Keith J., and Peter K. Snyder. "The Relationship between the Pacific–North American Teleconnection Pattern, the Great Plains Low-Level Jet, and North Central U.S. Heavy Rainfall Events*." Journal of Climate 28, no. 17 (2015): 6729–42. http://dx.doi.org/10.1175/jcli-d-14-00657.1.
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Abstract This study demonstrates the relationship between the Pacific–North American (PNA) teleconnection pattern and the Great Plains low-level jet (GPLLJ). The negative phase of the PNA, which is associated with lower heights over the Great Plains and ridging in the southeastern United States, enhances the GPLLJ by increasing the pressure gradient within the GPLLJ on 6-hourly to monthly time scales. Strong GPLLJ events predominantly occur when the PNA is negative. Warm-season strong GPLLJ events with a very negative PNA (<−1) are associated with more persistent, longer wavelength planetary waves that increase the duration of GPLLJ events and enhance precipitation over the north central United States. When one considers the greatest 5-day north central U.S. precipitation events, a large majority occur when the PNA is negative, with most exhibiting a very negative PNA. Stronger moisture transport during heavy rainfall events with a very negative PNA decreases the precipitation of locally derived moisture compared to events with a very positive PNA. The PNA becomes negative 2–12 days before heavy rainfall events and is very negative within two weeks of 78% of heavy rainfall events in the north central United States, a finding that could be used to improve medium-range forecasts of heavy rainfall events.
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Dai, Ying, Steven B. Feldstein, Benkui Tan, and Sukyoung Lee. "Formation Mechanisms of the Pacific–North American Teleconnection with and without Its Canonical Tropical Convection Pattern." Journal of Climate 30, no. 9 (2017): 3139–55. http://dx.doi.org/10.1175/jcli-d-16-0411.1.
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The mechanisms that drive the Pacific–North American (PNA) teleconnection pattern with and without its canonical tropical convection pattern are investigated with daily ERA-Interim and NOAA OLR data (the former pattern is referred to as the convective PNA, and the latter pattern is referred to as the nonconvective PNA). Both the convective and nonconvective positive PNA are found to be preceded by wave activity fluxes associated with a Eurasian wave train. These wave activity fluxes enter the central subtropical Pacific, a location that is favorable for barotropic wave amplification, just prior to the rapid growth of the PNA. The wave activity fluxes are stronger for the positive nonconvective PNA, suggesting that barotropic amplification plays a greater role in its development. The negative convective PNA is also preceded by a Eurasian wave train, whereas the negative nonconvective PNA grows from the North Pacific contribution to a circumglobal teleconnection pattern. Driving by high-frequency eddy vorticity fluxes is largest for the negative convective PNA, indicating that a positive feedback may be playing a more dominant role in its development. The lifetimes of convective PNA events are found to be longer than those of nonconvective PNA events, with the former (latter) persisting for about three (two) weeks. Furthermore, the frequency of the positive (negative) convective PNA is about 40% (60%) greater than that of the positive (negative) nonconvective PNA.
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Song, Jie. "Understanding Anomalous Synoptic Eddy Vorticity Forcing in Pacific–North American Teleconnection Pattern Events." Journal of the Atmospheric Sciences 75, no. 12 (2018): 4287–312. http://dx.doi.org/10.1175/jas-d-18-0071.1.
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Abstract Utilizing a decomposition of anomalous eddy vorticity forcing (EVF) proposed by Song in 2016 and a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model, this study provides a different understanding of physical mechanisms that are responsible for the formation of the anomalous synoptic EVF (SEVF) associated with Pacific–North American teleconnection pattern (PNA) events. A series of short-term control experiments (CEs) and initial-value modified experiments (IVMEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. IVMEs are exactly the same as CEs except that appropriate small perturbations are introduced into the initial-value fields of CEs. The modified initial-value fields led to a gradual development of the PNA-like flow anomalies in IVMEs. Based on these numerical results, deformations of the synoptic eddy due to the emergence of the PNA pattern can be easily acquired by subtracting the synoptic eddy in CEs from the synoptic eddy in IVMEs . The anomalous SEVF associated with the PNA events in the model can be decomposed into ensembles of two linear and interaction terms (EVF1 and EVF2) and a nonlinear self-interaction term (EVF3). It is demonstrated that the physical essence of the anomalous SEVF associated with the PNA events is a competition result between EVF1 plus EVF2 and EVF3. Results also indicate that the different signs of SEVF associated with the positive and negative PNA events are not necessarily related to the different tilts of the synoptic eddy.
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Baxter, Stephen, and Sumant Nigam. "A Subseasonal Teleconnection Analysis: PNA Development and Its Relationship to the NAO." Journal of Climate 26, no. 18 (2013): 6733–41. http://dx.doi.org/10.1175/jcli-d-12-00426.1.
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Abstract The Pacific–North American (PNA) teleconnection is a major mode of Northern Hemisphere wintertime climate variability, with well-known impacts on North American temperature and precipitation. To assess whether the PNA teleconnection has extended predictability, comprehensive data analysis is conducted to elucidate PNA evolution, with an emphasis on patterns of PNA development and decay. These patterns are identified using extended empirical orthogonal function (EEOF) and linear regression analyses on pentad-resolution atmospheric circulation data from the new Climate Forecast System Reanalysis (CFSR). Additionally, dynamical links between the PNA and another important mode of wintertime variability, the North Atlantic Oscillation (NAO), are analyzed both in the presence and absence of notable tropical convections, for example, the Madden–Julian oscillation (MJO), which is known to be influential on both. The relationship is analyzed using EEOF and regression techniques. It is shown that the PNA structure is similar in both space and time when the MJO is linearly removed from the dataset. Furthermore, there is a small but significant lag between the NAO and PNA, with the NAO leading a PNA of opposite phase on time scales of one to three pentads. It is suggested from barotropic vorticity analysis that this relationship may result in part from excitation of Rossby waves by the NAO in the Asian waveguide.
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Chu, J. E., A. Timmermann, and J. Y. Lee. "North American April tornado occurrences linked to global sea surface temperature anomalies." Science Advances 5, no. 8 (2019): eaaw9950. http://dx.doi.org/10.1126/sciadv.aaw9950.
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Annual tornado occurrences over North America display large interannual variability and a statistical linkage to sea surface temperature (SST) anomalies. However, the underlying physical mechanisms for this connection and its modulation in a rapidly varying seasonal environment still remain elusive. Using tornado data over the United States from 1954 to 2016 in combination with SST-forced atmospheric general circulation models, we show a robust dynamical linkage between global SST conditions in April, the emergence of the Pacific-North American teleconnection pattern (PNA), and the year-to-year tornado activity in the Southern Great Plains (SGP) region of the United States. Contrasting previous studies, we find that only in April SST-driven atmospheric circulation anomalies can effectively control the northward moisture-laden flow from the Gulf of Mexico, boosting low-level moisture flux convergence over the SGP. These strong large-scale connections are absent in other months because of the strong seasonality of the PNA and background moisture conditions.
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Loikith, Paul C., and Anthony J. Broccoli. "The Influence of Recurrent Modes of Climate Variability on the Occurrence of Winter and Summer Extreme Temperatures over North America." Journal of Climate 27, no. 4 (2014): 1600–1618. http://dx.doi.org/10.1175/jcli-d-13-00068.1.
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Abstract The influence of the Pacific–North American (PNA) pattern, the northern annular mode (NAM), and the El Niño–Southern Oscillation (ENSO) on extreme temperature days and months over North America is examined. Associations between extreme temperature days and months are strongest with the PNA and NAM and weaker for ENSO. In general, the association with extremes tends to be stronger on monthly than daily time scales and for winter as compared to summer. Extreme temperatures are associated with the PNA and NAM in the vicinity of the centers of action of these circulation patterns; however, many extremes also occur on days when the amplitude and polarity of these patterns do not favor their occurrence. In winter, synoptic-scale, transient weather disturbances are important drivers of extreme temperature days; however, many of these smaller-scale events are concurrent with amplified PNA or NAM patterns. Associations are weaker in summer when other physical mechanisms affecting the surface energy balance, such as anomalous soil moisture content, also influence the occurrence of extreme temperatures.
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Zhu, Zhiwei, and Tim Li. "A New Paradigm for Continental U.S. Summer Rainfall Variability: Asia–North America Teleconnection." Journal of Climate 29, no. 20 (2016): 7313–27. http://dx.doi.org/10.1175/jcli-d-16-0137.1.
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Abstract The present study reveals a close relationship between the leading mode of continental U.S. (CONUS) summer rainfall and the East Asian subtropical monsoon rainfall (viz., mei-yu in China, baiu in Japan, and changma in the Korean peninsula). The East Asian subtropical monsoon rainfall and the CONUS dipole rainfall patterns are connected by an upper-level Asia–North America (ANA) teleconnection. The Rossby wave energy propagates along the path of the westerly jet stream (WJS) from East Asia to North America, affecting the CONUS summer rainfall. Mechanisms through which East Asian summer monsoon heating influence North American rainfall are illustrated by idealized anomaly atmospheric general circulation model experiments. In boreal winter, because of the southward shift of the WJS, the Pacific–North American (PNA) pattern can be excited by the tropical central/eastern Pacific heating associated with El Niño, affecting the rainfall over CONUS. In boreal summer, because the WJS is weaker and locates farther to the north, an equatorial heating anomaly cannot directly perturb the WJS. A perturbation heating over subtropical East Asia, however, can trigger an ANA pattern along the path of the WJS, affecting the rainfall over North America. The season-dependent teleconnection scenario illustrates that the predictability source of CONUS rainfall variability is different between winter and summer. While the PNA pattern generated by El Niño is critical for CONUS rainfall in northern winter, the CONUS dipole rainfall variation in boreal summer is mainly governed by the remote forcing over subtropical East Asia via the ANA teleconnection.
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Chien, Yu-Tang, S. Y. Simon Wang, Yoshimitsu Chikamoto, Steve L. Voelker, Jonathan D. D. Meyer, and Jin-Ho Yoon. "North American Winter Dipole: Observed and Simulated Changes in Circulations." Atmosphere 10, no. 12 (2019): 793. http://dx.doi.org/10.3390/atmos10120793.
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In recent years, a pair of large-scale circulation patterns consisting of an anomalous ridge over northwestern North America and trough over northeastern North America was found to accompany extreme winter weather events such as the 2013–2015 California drought and eastern U.S. cold outbreaks. Referred to as the North American winter dipole (NAWD), previous studies have found both a marked natural variability and a warming-induced amplification trend in the NAWD. In this study, we utilized multiple global reanalysis datasets and existing climate model simulations to examine the variability of the winter planetary wave patterns over North America and to better understand how it is likely to change in the future. We compared between pre- and post-1980 periods to identify changes to the circulation variations based on empirical analysis. It was found that the leading pattern of the winter planetary waves has changed, from the Pacific–North America (PNA) mode to a spatially shifted mode such as NAWD. Further, the potential influence of global warming on NAWD was examined using multiple climate model simulations.
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Ault, Toby R., Alison K. Macalady, Gregory T. Pederson, Julio L. Betancourt, and Mark D. Schwartz. "Northern Hemisphere Modes of Variability and the Timing of Spring in Western North America." Journal of Climate 24, no. 15 (2011): 4003–14. http://dx.doi.org/10.1175/2011jcli4069.1.
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Abstract Spatial and temporal patterns of variability in spring onset are identified across western North America using a spring index (SI) model based on weather station minimum and maximum temperatures (Tmin and Tmax, respectively). Principal component analysis shows that two significant and independent patterns explain roughly half of the total variance in the timing of spring onset from 1920 to 2005. However, these patterns of spring onset do not appear to be linear responses to the primary modes of variability in the Northern Hemisphere: the Pacific–North American pattern (PNA) and the northern annular mode (NAM). Instead, over the period when reanalysis data and the spring index model overlap (1950–2005), the patterns of spring onset are local responses to the state of both the PNA and NAM, which together modulate the onset date of spring by 10–20 days on interannual time scales. They do so by controlling the number and intensity of warm days. There is also a regionwide trend in spring advancement of about −1.5 days decade−1 from 1950 to 2005. Trends in the NAM and PNA can only explain about one-third (−0.5 day decade−1) of this trend.
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Bai, Xuezhi, Jia Wang, Qinzheng Liu, Dongxiao Wang, and Yu Liu. "Severe Ice Conditions in the Bohai Sea, China, and Mild Ice Conditions in the Great Lakes during the 2009/10 Winter: Links to El Niño and a Strong Negative Arctic Oscillation." Journal of Applied Meteorology and Climatology 50, no. 9 (2011): 1922–35. http://dx.doi.org/10.1175/2011jamc2675.1.
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AbstractThis study investigates the causes of severe ice conditions over the Bohai Sea, China, and mild ice cover over the North American Great Lakes under the same hemispheric climate patterns during the 2009/10 winter with a strong negative Arctic Oscillation (AO) and an El Niño event. The main cause of severe ice cover over the Bohai Sea was the strong negative AO. Six of seven winters with severe ice were associated with a strong negative AO during the period 1954–2010. The Siberian high (SH) in the 2009/10 winter was close to normal. The influence of El Niño on the Bohai Sea was not significant. In contrast, the mild ice conditions in the Great Lakes were mainly caused by the strong El Niño event. Although the negative AO generally produces significant colder surface air temperature (SAT) and heavy ice cover over the Great Lakes, when it coincided with a strong El Niño event during the 2009/10 winter the El Niño–induced Pacific–North America (PNA)-like pattern dominated the midlatitudes and was responsible for the flattening of the ridge–trough system over North America, leading to warmer-than-normal temperatures and mild ice conditions over the Great Lakes. This comparative study revealed that interannual variability of SAT in North America, including the Great Lakes, is effectively influenced by El Niño events through a PNA or PNA-like pattern whereas the interannual variability of SAT in northeastern China, including the Bohai Sea area, was mainly controlled by AO and SH.
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Sugimoto, Shusaku, and Kimio Hanawa. "The Wintertime Wind Stress Curl Field in the North Atlantic and Its Relation to Atmospheric Teleconnection Patterns." Journal of the Atmospheric Sciences 67, no. 5 (2010): 1687–94. http://dx.doi.org/10.1175/2009jas3339.1.
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Abstract Adopting a rotated empirical orthogonal function (REOF) analysis and a maximum covariance analysis (MCA), characteristics of the wintertime wind stress curl (WSC) anomaly field in the North Atlantic are investigated. In terms of both temporal variation and spatial distribution, the first four leading modes of WSC show a one-to-one relation with four atmospheric teleconnection patterns over the North Atlantic sector: the North Atlantic Oscillation (NAO) and the east Atlantic (EA), tropical–Northern Hemisphere (TNH), and Pacific–North American (PNA) patterns. These four patterns characterize the WSC variations over the different regions in the North Atlantic: NAO and EA over the eastern side of the basin, TNH over the central part of the basin, and PNA over the western side of the basin.
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Dai, Ying, and Benkui Tan. "Two Types of the Western Pacific Pattern, Their Climate Impacts, and the ENSO Modulations." Journal of Climate 32, no. 3 (2019): 823–41. http://dx.doi.org/10.1175/jcli-d-17-0618.1.
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The western Pacific (WP) pattern is a major teleconnection pattern that influences the wintertime Northern Hemisphere climate variations. Based on daily NCEP–NCAR reanalysis data, this study examines the climate impacts and the El Niño–Southern Oscillation (ENSO) modulation of two types of the WP pattern. The result shows that the WP patterns may arise from precursory disturbances over Asia and the North Pacific or from the Pacific–North American (PNA) pattern of the same polarity as or opposite polarity to that of the WP patterns. Among these WP patterns, the WP patterns that arise from the PNA pattern of the same polarity are most influential on North American near-surface and polar stratospheric air temperatures; furthermore, their frequency of occurrence, amplitude, and duration can be affected by ENSO phases: the positive WP patterns occur more frequently with larger amplitude and longer duration in El Niño than in La Niña; and the negative WP patterns occur less frequently with smaller amplitude and shorter duration in El Niño than in La Niña. The above findings suggest that the PNA pattern plays a crucial role in the climate impacts and the ENSO modulation of the WP patterns.
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Eichler, Timothy Paul, and Jon Gottschalck. "Interannual Variability of Northern Hemisphere Storm Tracks in Coarse-Gridded Datasets." Advances in Meteorology 2013 (2013): 1–15. http://dx.doi.org/10.1155/2013/545463.
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Extratropical cyclones exert a large socioeconomic impact. It is therefore important to assess their interannual variability. We generate cyclone tracks from the National Center for Environmental Prediction’s Reanalysis I and the European Centre for Medium Range Prediction ERA-40 reanalysis datasets. To investigate the interannual variability of cyclone tracks, we compare the effects of El Niño, the North Atlantic Oscillation (NAO), the Indian Ocean Dipole (IOD), and the Pacific North American Pattern (PNA) on cyclone tracks. Composite analysis shows similar results for the impacts of El Niño, NAO, and the PNA on NH storm tracks. Although it is encouraging, we also found regional differences when comparing reanalysis datasets. The results for the IOD suggested a wave-like alteration of cyclone frequency across the northern US/Canada possibly related to Rossby wave propagation. Partial correlation demonstrates that although El Niño affects cyclone frequency in the North Pacific and along the US east coast, its impact on the North Pacific is accomplished via the PNA. Similarly, the PNA’s impact on US east coast storms is modulated via El Niño. In contrast, the impacts of the NAO extend as far west as the North Pacific and are not influenced by either the PNA or El Niño.
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Liu, Zhongfang, Kei Yoshmura, Gabriel J. Bowen та Jeffrey M. Welker. "Pacific–North American Teleconnection Controls on Precipitation Isotopes (δ18O) across the Contiguous United States and Adjacent Regions: A GCM-Based Analysis". Journal of Climate 27, № 3 (2014): 1046–61. http://dx.doi.org/10.1175/jcli-d-13-00334.1.
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Abstract The Pacific–North American (PNA) teleconnection pattern has a strong influence on North America’s winter climate, but much less is known about how the PNA pattern controls precipitation isotopes (e.g., δ18O) across the United States. In this study, an isotopically equipped atmospheric general circulation model (isoGSM) is used to investigate how divergent phases of the PNA affect precipitation δ18O values across the United States. A simulation using observational climate and isotope data over the United States is evaluated first. The simulation explains 84% of the spatial variability of winter precipitation δ18O, with an overestimation in the northern Rocky Mountains and the Great Lakes. Temporally, the simulation explains 29%–81% of the interannual variability of winter precipitation δ18O, with typically a higher explained variance in the east than the west. The modeled winter precipitation δ18O exhibits a clear northwest–southeast (NW–SE) dipolelike pattern in response to shifts in the PNA pattern, with the center of positive polarity in the northwestern United States and the Canadian prairies and the center of negative polarity over the Ohio River valley. This dipolelike spatial pattern is a result of the difference in atmospheric circulation and moisture sources associated with the PNA pattern. These results highlight the importance of the PNA-associated circulation dynamics in governing precipitation isotope patterns across the United States. This understanding improves our ability to interpret paleoclimate records of water isotope/hydrologic change across the United States with a much greater appreciation of regional traits. The robust antiphase oscillation in precipitation isotopes in response to shifting the PNA pattern provides a promising opportunity to reconstruct the past variability in the PNA pattern that may be recorded in ice cores, tree rings, lake sediments, and speleothems.
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Shabbar, A. "The impact of El Niño-Southern Oscillation on the Canadian climate." Advances in Geosciences 6 (January 30, 2006): 149–53. http://dx.doi.org/10.5194/adgeo-6-149-2006.
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Abstract. The quasi-periodic El Niño -Southern Oscillation (ENSO) phenomenon in the tropical Pacific Ocean produces the largest interannual variation in the cold season climate of Canada. The diabatic heating in the eastern tropical Pacific, associated with the warm phase of ENSO (El Niño), triggers Rossby waves which in turn gives rise to the Pacific-North American teleconnection (PNA) over the North American sector. The strongest cell of the PNA pattern lies over western Canada. In most of southern Canada, mean winter temperature distribution is shifted towards warmer values, and precipitation is below normal. The presence of El Niño provides the best opportunity to make skillful long-range winter forecast for Canada. A strong El Niño event, while bringing respite from the otherwise cold winter in Canada, can be expected to cost the Canadian economy two to five billion dollars.
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Archambault, Heather M., Lance F. Bosart, Daniel Keyser, and Anantha R. Aiyyer. "Influence of Large-Scale Flow Regimes on Cool-Season Precipitation in the Northeastern United States." Monthly Weather Review 136, no. 8 (2008): 2945–63. http://dx.doi.org/10.1175/2007mwr2308.1.
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Abstract The influence of large-scale flow regimes on cool-season (November–April) northeastern U.S. (Northeast) precipitation is investigated for the period 1948–2003 from statistical and synoptic perspectives. These perspectives are addressed through (i) a statistical analysis of cool-season Northeast precipitation associated with the North Atlantic Oscillation (NAO) and Pacific–North American (PNA) regimes (one standard deviation or greater NAO or PNA daily index anomalies persisting several days), and (ii) a composite analysis of the synoptic signatures of major (two standard deviation) 24-h cool-season Northeast precipitation events occurring during NAO and PNA regimes. The statistical analysis reveals that negative PNA regimes are associated with above-average cool-season Northeast precipitation and an above-average frequency of light and moderate precipitation events, whereas the opposite associations are true for positive PNA regimes. In comparison with PNA regimes, NAO regimes are found to have relatively little influence on the amount and frequency of cool-season Northeast precipitation. The composite analysis indicates that a surface cyclone flanked by an upstream trough over the Ohio Valley and downstream ridge over eastern Canada and upper- and lower-level jets in the vicinity of the Northeast are characteristic signatures of major cool-season Northeast precipitation events occurring during NAO and PNA regimes. Negative NAO and positive PNA precipitation events, however, are associated with a more amplified trough–ridge pattern and greater implied Atlantic moisture transport by a low-level jet into the Northeast than positive NAO and negative PNA precipitation events. Furthermore, a signature of lateral upper-level jet coupling is noted only during positive and negative PNA precipitation events.
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Notaro, M., W.-C. Wang, and W. Gong. "Model and Observational Analysis of the Northeast U.S. Regional Climate and Its Relationship to the PNA and NAO Patterns during Early Winter." Monthly Weather Review 134, no. 11 (2006): 3479–505. http://dx.doi.org/10.1175/mwr3234.1.
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Abstract The relationship between the large-scale circulation and regional climate of the northeast United States is investigated for early winter using observational data and the State University of New York at Albany regional climate model. Simulated patterns of temperature, precipitation, and atmospheric circulation compare well with observations, despite a cold, dry bias. Ten December runs are analyzed to investigate the impact of the Pacific–North American (PNA) pattern on temperature, precipitation, clouds, and circulation features. During a positive PNA pattern, the simulated and observed eastern U.S. jet shifts to the southeast, coinciding with cold, dry conditions in the Northeast. This shift and intensification of the upper-level jet stream during a positive PNA pattern coincides with a greater frequency of cyclones and anticyclones along a distinct southwest–northeast track. Despite increased cyclone activity, total wintertime precipitation is below normal during a positive PNA pattern because of enhanced stability and subsidence over land, along with lower-atmospheric moisture content. Lower surface air temperatures during a positive PNA pattern result in enhanced simulated cloud cover over the Great Lakes and Atlantic Ocean due to increased thermal contrast and fluxes of sensible and latent heat, and a reduction in clouds over land. Interactions between the PNA and North Atlantic Oscillation (NAO) patterns impact the Northeast winter climate. Observed frontal passages through New York are most abundant during a negative PNA and positive NAO pattern, with a zonal upper-level jet positioned over New York. A positive PNA pattern is frequently characterized by an earlier observed Great Lakes ice season, while the greatest lake-effect snowfall occurs during a positive PNA and negative NAO pattern. The NAO pattern has the largest impact on northeast U.S. temperatures and the eastern U.S. upper-level jet during a positive PNA pattern.
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Tseng, Kai-Chih, Nathaniel C. Johnson, Eric D. Maloney, Elizabeth A. Barnes, and Sarah B. Kapnick. "Mapping Large-Scale Climate Variability to Hydrological Extremes: An Application of the Linear Inverse Model to Subseasonal Prediction." Journal of Climate 34, no. 11 (2021): 4207–25. http://dx.doi.org/10.1175/jcli-d-20-0502.1.
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AbstractThe excitation of the Pacific–North American (PNA) teleconnection pattern by the Madden–Julian oscillation (MJO) has been considered one of the most important predictability sources on subseasonal time scales over the extratropical Pacific and North America. However, until recently, the interactions between tropical heating and other extratropical modes and their relationships to subseasonal prediction have received comparatively little attention. In this study, a linear inverse model (LIM) is applied to examine the tropical–extratropical interactions. The LIM provides a means of calculating the response of a dynamical system to a small forcing by constructing a linear operator from the observed covariability statistics of the system. Given the linear assumptions, it is shown that the PNA is one of a few leading modes over the extratropical Pacific that can be strongly driven by tropical convection while other extratropical modes present at most a weak interaction with tropical convection. In the second part of this study, a two-step linear regression is introduced that leverages a LIM and large-scale climate variability to the prediction of hydrological extremes (e.g., atmospheric rivers) on subseasonal time scales. Consistent with the findings of the first part, most of the predictable signals on subseasonal time scales are determined by the dynamics of the MJO–PNA teleconnection while other extratropical modes are important only at the shortest forecast leads.
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Itoh, Hisanori. "Reconsideration of the True versus Apparent Arctic Oscillation." Journal of Climate 21, no. 10 (2008): 2047–62. http://dx.doi.org/10.1175/2007jcli2167.1.
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Abstract The physical reality of the Arctic Oscillation (AO; or northern annular mode) is considered. The data used are mainly the monthly mean sea level pressure (SLP). A schematic figure is first presented to illustrate the relationship between the North Atlantic Oscillation (NAO)–Pacific–North American Oscillation (PNA) system and the AO–negative correlation mode between the Atlantic and the Pacific (AO–NCM) system. Although the NAO–PNA (apparent AO–NCM) and true AO–NCM systems give rise to the same EOFs, the probability density functions for the time coefficients of the two leading modes are different. Therefore, the discrimination of the two systems is possible. Several pieces of evidence indicate that, in the real world, the NAO–PNA and the AO–NCM are located on almost the same plane in phase space. This means that the NAO–PNA and AO–NCM systems have the same variations on the plane in common, implying that when the NAO–PNA system is real, the AO–NCM is unlikely to be real. Simple independent component analysis is carried out to distinguish between the true and apparent AO–NCM systems, indicating that the NAO and PNA are independent oscillations, that is, true ones. The analysis is extended to the winter mean SLP field, for which the EOF shows the NAO–PNA but not the AO–NCM. This may be due to the fact that the winter mean NAO and PNA patterns have little spatial correlation. Calculations using randomly selected samples also indicate that when the NAO and PNA patterns have little spatial correlation, the AO never appears as EOF1. All the preceding results show that almost all characteristics of the AO–NCM can be explained from those of the NAO–PNA. Hence it is concluded that the AO, which is extracted by EOF analysis from the temporarily independent but spatially overlapping variations of the NAO and PNA, is almost apparent.
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Vorobyeva, Vasilisa V., and Evgenii M. Volodin. "Analysis of the predictability of stratospheric variability and climate indices based on seasonal retrospective forecasts of the INM RAS climate model." Russian Journal of Numerical Analysis and Mathematical Modelling 36, no. 2 (2021): 117–26. http://dx.doi.org/10.1515/rnam-2021-0010.
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Abstract Ensemble numerical experiments for winter seasons of 1980–2014 were carried out with the use of the mathematical climate model of the Institute of Numerical Mathematics (INM) of the Russian Academy of Sciences developed initially for multi-year climate forecasts. Based on the results obtained in this research, a qualitative assessment of the reproduction of the North Atlantic (NAO) and Pacific-North American (PNA) oscillation indiceswas obtained. It was shown that the INM-CM5-0 climate model has a very high predictability of the winter NAO index and one, but not unique reason for this is the predictability of the stratospheric variability in the INM RAS model. The analysis of the quality of reproduction of the PNA index on a seasonal time scale for the INM-CM5-0 model has shown an acceptable result.
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Frankignoul, Claude, and Nathalie Sennéchael. "Observed Influence of North Pacific SST Anomalies on the Atmospheric Circulation." Journal of Climate 20, no. 3 (2007): 592–606. http://dx.doi.org/10.1175/jcli4021.1.
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Abstract A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K−1 in the MCA and 20 m K−1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K−1 in the MCA and about 35 m K−1 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.
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Archambault, Heather M., Daniel Keyser, and Lance F. Bosart. "Relationships between Large-Scale Regime Transitions and Major Cool-Season Precipitation Events in the Northeastern United States." Monthly Weather Review 138, no. 9 (2010): 3454–73. http://dx.doi.org/10.1175/2010mwr3362.1.
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Abstract This observational study investigates statistical and synoptic–dynamic relationships between regime transitions, defined as a North Atlantic Oscillation (NAO) or Pacific–North American pattern (PNA) index change from at least a 1 standard deviation anomaly to at least a 1 standard deviation anomaly of opposite sign within 7 days, and cool-season (November–April) northeastern U.S. (NE) precipitation. A statistical analysis is performed of daily cool-season NE precipitation during all NAO and PNA transitions for 1948–2003, and a composite analysis and case study of a major cool-season NE precipitation event occurring during a positive-to-negative NAO transition are conducted. Datasets used are the 0.25° NCEP Unified Precipitation Dataset, the 2.5° NCEP–NCAR reanalysis, and the 1.125° 40-yr ECMWF Re-Analysis (ERA-40). Results of the statistical analysis suggest that cool-season NE precipitation tends to be enhanced during positive-to-negative NAO and negative-to-positive PNA transitions, and suppressed during negative-to-positive NAO and positive-to-negative PNA transitions. Of the four types of regime transitions, only the positive-to-negative NAO transition is associated with substantially more frequent major cool-season NE precipitation events compared to climatology. Results of the composite analysis and case study indicate that a surface cyclone and cyclonic wave breaking associated with the major NE precipitation event can help produce a high-latitude blocking pattern over the North Atlantic characteristic of a negative NAO pattern via thermal advection, potential vorticity transport, and diabatic processes.
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Ewen, Tracy, Stefan Brönnimann, and Jeffrey Annis. "An Extended Pacific–North American Index from Upper-Air Historical Data Back to 1922." Journal of Climate 21, no. 6 (2008): 1295–308. http://dx.doi.org/10.1175/2007jcli1951.1.
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Abstract This paper presents a reconstruction of a Pacific–North America (PNA) index from historical upper-level data for the period 1922–47. The data have been compiled from a number of sources and cover the Pacific–North American sector relatively well over this time period. Temperature and geopotential height profiles from aircraft, kite, and radiosonde ascents back to 1922 have been digitized and validated. Wind speed and direction from pilot balloon data back to the early 1920s, provided by NCAR, have also been used. A statistical regression approach is used for the reconstruction and calibrated in the post-1948 period using NCEP–NCAR reanalysis data. Split-sample validation experiments were performed within the NCEP–NCAR period, and sensitivity experiments with different subsets of predictors were performed. Similar reconstructions and validation experiments were carried out using a 540-yr control run from the Community Climate System Model, version 3 (CCSM3). The reconstructed index series together with validation statistics for both the historical and model data are presented. Excellent reconstruction skill is found for the winter months, while the reconstructions are somewhat worse in summer. Compared with a reconstruction based only on surface data, the addition of the newly digitized upper-air stations improves the reconstruction skill in all seasons. The historical reconstruction is presented with respect to its imprint on hemispheric fields of surface air temperature, sea level pressure, and precipitation with a special focus on extreme cases. In addition, the extended PNA index is compared with indices of the North Atlantic Oscillation, the Pacific decadal oscillation, and the El Niño–Southern Oscillation. The relationship to these indices is found to be stationary over the analysis period.
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Bai, Xuezhi, and Jia Wang. "Atmospheric teleconnection patterns associated with severe and mild ice cover on the Great Lakes, 1963–2011." Water Quality Research Journal 47, no. 3-4 (2012): 421–35. http://dx.doi.org/10.2166/wqrjc.2012.009.
Full textAbstract:
Atmospheric teleconnection circulation patterns associated with severe and mild ice cover over the Great Lakes are investigated using the composite analysis of lake ice data and National Center of Environmental Prediction (NCEP) reanalysis data for the period 1963–2011. The teleconnection pattern associated with the severe ice cover is the combination of a negative North Atlantic Oscillation (NAO) or Arctic Oscillation (AO) and negative phase of Pacific/North America (PNA) pattern, while the pattern associated with the mild ice cover is the combination of a positive PNA (or an El Niño) and a positive phase of the NAO/AO. These two extreme ice conditions are associated with the North American ridge–trough variations. The intensified ridge–trough system produces a strong northwest-to-southeast tilted ridge and trough and increases the anomalous northwesterly wind, advecting cold, dry Arctic air to the Great Lakes. The weakened ridge–trough system produces a flattened ridge and trough, and promotes a climatological westerly wind, advecting warm, dry air from western North America to the Great Lakes. Although ice cover for all the individual lakes responds roughly linearly and symmetrically to both phases of the NAO/AO, and roughly nonlinearly and asymmetrically to El Niño and La Niña events, the overall ice cover response to individual NAO/AO or Niño3.4 index is not statistically significant. The combined NAO/AO and Niño3.4 indices can be used to reliably project severe ice cover during the simultaneous –NAO/AO and La Niña events, and mild ice cover during the simultaneous +NAO/AO and El Niño events.
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Li, Wei, and Chris E. Forest. "Estimating the Sensitivity of the Atmospheric Teleconnection Patterns to SST Anomalies Using a Linear Statistical Method." Journal of Climate 27, no. 24 (2014): 9065–81. http://dx.doi.org/10.1175/jcli-d-14-00231.1.
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Abstract The Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO) are known to contain a tropical sea surface temperature (SST)-forced component. This study examines the sensitivity of the wintertime NAO and PNA to patterns of tropical SST anomalies using a linear statistical–dynamic method. The NAO index is sensitive to SST anomalies over the tropical Indian Ocean, the central Pacific Ocean, and the Caribbean Sea, and the PNA index is sensitive to SST anomalies over the tropical Pacific and Indian Oceans. The NAO and PNA patterns can be reproduced well by combining the linear operator with the consistent SST anomaly over the Indian Ocean and the Niño-4 regions, respectively, suggesting that these are the most efficient ocean basins that force the teleconnection patterns. During the period of 1950–2000, the NAO time series reconstructed by using SST anomalies over the Indian Ocean + Niño-4 region + Caribbean Sea or the Indian Ocean + Niño-4 region is significantly correlated with the observation. Using a cross-spectral analysis, the NAO index is coherent with the SST forcing over the Indian Ocean at a significant 3-yr period and a less significant 10-yr period, with the Indian Ocean SST leading by about a quarter phase. Unsurprisingly, the PNA index is most coherent with the Niño-4 SST at 4–5-yr periods. When compared with the observation, the NAO variability from the linear reconstruction is better reproduced than that of the coupled model, which is better than the Atmospheric Model Intercomparison Project (AMIP) run, while the PNA variability from the AMIP simulations is better than that of the reconstruction, which is better than the coupled model run.
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Taguchi, Bunmei, Hisashi Nakamura, Masami Nonaka, et al. "Seasonal Evolutions of Atmospheric Response to Decadal SST Anomalies in the North Pacific Subarctic Frontal Zone: Observations and a Coupled Model Simulation." Journal of Climate 25, no. 1 (2012): 111–39. http://dx.doi.org/10.1175/jcli-d-11-00046.1.
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Abstract Potential impacts of pronounced decadal-scale variations in the North Pacific sea surface temperature (SST) that tend to be confined to the subarctic frontal zone (SAFZ) upon seasonally varying atmospheric states are investigated, by using 48-yr observational data and a 120-yr simulation with an ocean–atmosphere coupled general circulation model (CGCM). SST fields based on in situ observations and the ocean component of the CGCM have horizontal resolutions of 2.0° and 0.5°, respectively, which can reasonably resolve frontal SST gradient across the SAFZ. Both the observations and CGCM simulation provide a consistent picture between SST anomalies in the SAFZ yielded by its decadal-scale meridional displacement and their association with atmospheric anomalies. Correlated with SST anomalies persistent in the SAFZ from fall to winter, a coherent decadal-scale signal in the wintertime atmospheric circulation over the North Pacific starts emerging in November and develops into an equivalent barotropic anomaly pattern similar to the Pacific–North American (PNA) pattern. The PNA-like signal with the weakened (enhanced) surface Aleutian low correlated with positive (negative) SST anomalies in the SAFZ becomes strongest and most robust in January, under the feedback forcing from synoptic-scale disturbances migrating along the Pacific storm track that shifts northward (southward) in accord with the oceanic SAFZ. This PNA-like signal, however, breaks down in February, which is suggestive of a particular sensitivity of that anomaly pattern to subtle differences in the background climatological-mean state. Despite its collapse in February, the PNA-like signal recurs the next January. This subseasonal evolution of the signal suggests that the PNA-like anomaly pattern may develop as a response to the persistent SST anomalies that are maintained mainly through ocean dynamics.
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Goodrick, Scott L., and Deborah E. Hanley. "Florida wildfire activity and atmospheric teleconnections." International Journal of Wildland Fire 18, no. 4 (2009): 476. http://dx.doi.org/10.1071/wf07034.
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Since 1991, the Florida Division of Forestry has been making seasonal fire severity forecasts based on a relationship between area burned in Florida and El Niño–Southern Oscillation (ENSO). The present study extends the original analysis on which these forecasts are based and attempts to augment it with the addition of other patterns of climate variability. Two atmospheric teleconnection patterns, the North Atlantic Oscillation and Pacific–North American pattern, are examined as potential indicators of seasonal and monthly area burned in Florida. Although ENSO was the only climate index to show a significant correlation to area burned in Florida, the Pacific–North American pattern (PNA) is shown to be a factor influencing fire season severity although the relationship is not monotonic and therefore not revealed by correlation analysis.
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Henderson, Stephanie A., Daniel J. Vimont, and Matthew Newman. "The Critical Role of Non-Normality in Partitioning Tropical and Extratropical Contributions to PNA Growth." Journal of Climate 33, no. 14 (2020): 6273–95. http://dx.doi.org/10.1175/jcli-d-19-0555.1.
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AbstractThe Pacific–North American (PNA) teleconnection pattern has been linked both to tropical phenomena, including the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO), and to internal extratropical processes, including interactions with the zonally varying basic state and synoptic eddies. Many questions remain, however, concerning how these various relationships act, both separately and together, to yield observed PNA variability. Using linear inverse modeling (LIM), this study finds that the development and amplification of PNA anomalies largely results from the interference of modes strongly coupled to sea surface temperatures (SST), such as ENSO, and modes internal to the atmosphere, including the MJO. These SST-coupled and “internal atmospheric” modes form subspaces that are not orthogonal, and PNA growth is shown to occur via non-normal interactions. An internal atmospheric space LIM is developed to examine growth beyond this interference by removing the SST-coupled modes, effectively removing ENSO and retaining MJO variability. Optimal PNA growth in the internal atmospheric space LIM is driven by MJO heating, particularly over the Indian Ocean, and a retrograding northeast Pacific streamfunction anomaly. Additionally, the individual contributions of tropical heating and the extratropical circulation on PNA growth are investigated. The non-normal PNA growth is an important result, demonstrating the difficulty in partitioning PNA variance into contributions from different phenomena. This cautionary result is likely applicable to many geophysical phenomena and should be considered in attribution studies.
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Johansson, Åke. "Prediction Skill of the NAO and PNA from Daily to Seasonal Time Scales." Journal of Climate 20, no. 10 (2007): 1957–75. http://dx.doi.org/10.1175/jcli4072.1.
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Abstract The skill of state-of-the-art operational dynamical models in predicting the two most important modes of variability in the Northern Hemisphere extratropical atmosphere, the North Atlantic Oscillation (NAO) and Pacific–North American (PNA) teleconnection patterns, is investigated at time scales ranging from daily to seasonal. Two uncoupled atmospheric models used for deterministic forecasting in the short to medium range as well as eight fully coupled atmosphere–land–ocean forecast models used for monthly and seasonal forecasting are examined and compared. For the short to medium range, the level of forecast skill for the two indices is higher than that for the entire Northern Hemisphere extratropical flow. The forecast skill of the PNA is higher than that of the NAO. The forecast skill increases with the magnitude of the NAO and PNA indices, but the relationship is not pronounced. The probability density function (PDF) of the NAO and PNA indices is negatively skewed, in agreement with the distribution of skewness of the geopotential field. The models maintain approximately the observed PDF, including the negative skewness, for the first week. Extreme negative NAO/PNA events have larger absolute values than positive extremes in agreement with the negative skewness of the two indices. Recent large extreme events are generally well forecasted by the models. On the intraseasonal time scale it is found that both NAO and PNA have lingering forecast skill, in contrast to the Northern Hemisphere extratropical flow as a whole. This fact offers some hope for extended range forecasting, even though the skill is quite low. No conclusive positive benefit is seen from using higher horizontal resolution or coupling to the oceans. On the monthly and seasonal time scales, the level of forecast skill for the two indices is generally quite low, with the exception of winter predictions at short lead times.
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Rao, Jian, Chaim I. Garfinkel, and Rongcai Ren. "Modulation of the Northern Winter Stratospheric El Niño–Southern Oscillation Teleconnection by the PDO." Journal of Climate 32, no. 18 (2019): 5761–83. http://dx.doi.org/10.1175/jcli-d-19-0087.1.
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Using the CMIP5 multimodel ensemble (MME) historical experiments, the modulation of the stratospheric El Niño–Southern Oscillation (ENSO) teleconnection by the Pacific decadal oscillation (PDO) is investigated in this study. El Niño (La Niña) significantly impacts the extratropical stratosphere mainly during the positive (negative) PDO in the MME. Although the composite tropical ENSO SST intensities are similar during the positive and negative PDO in models, the Pacific–North American (PNA) responses are only significant when the PDO and ENSO are in phase. The local SST anomalies in the North Pacific can constructively (destructively) interfere with the tropical ENSO forcing to influence the extratropical eddy height anomalies when the PDO and ENSO are in (out of) phase. The difference between the positive and negative PDO in El Niño or La Niña winters filters out the tropical SST forcing, permitting the deduction of the extratropical SST contribution to the atmospheric response. The composite shows that the cold (warm) SST anomalies in the central North Pacific associated with the positive (negative) PDO have a similar impact to that of the warm (cold) SST anomalies in the tropical Pacific, exhibiting a positive (negative) PNA-like response, enhancing (weakening) the upward propagation of waves over the western coast of North America. The composite difference between the positive and negative PDO in El Niño or La Niña winters, as well as in eastern Pacific ENSO or central Pacific ENSO winters, presents a highly consistent atmospheric response pattern, which may imply a linear interference of the PDO’s impact with ENSO’s.
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Firing, Yvonne L., Mark A. Merrifield, Thomas A. Schroeder, and Bo Qiu. "Interdecadal Sea Level Fluctuations at Hawaii." Journal of Physical Oceanography 34, no. 11 (2004): 2514–24. http://dx.doi.org/10.1175/jpo2636.1.
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Abstract Over the past century, tide gauges in Hawaii have recorded interdecadal sea level variations that are coherent along the island chain. The generation of this signal and its relationship to other interdecadal variability are investigated, with a focus on the last decade. Hawaii sea level is correlated with sea surface height (SSH) over a significant portion of the North Pacific Ocean, and with the Pacific–North America (PNA) index, which represents teleconnections between tropical and midlatitude atmospheric variations. Similar variations extend well below the thermocline in World Ocean Atlas temperature. Comparison with NCEP reanalysis wind and pressure shows that high (low) sea level phases around Hawaii are associated with an increase (decrease) in the strength of the Aleutian low. The associated wind stress curl pattern is dynamically consistent with observed sea level anomalies, suggesting that sea level at Hawaii represents large-scale changes that are directly wind-forced in concert with the PNA. Atmospheric modulation, as opposed to Rossby wave propagation, may explain the linkage of Hawaii sea level to North American sea level and ENSO events. A wind-forced, baroclinic Rossby wave model replicates some aspects of the interdecadal SSH variations and their spatial structure but fails to predict them in detail near Hawaii. The accuracy of wind products in this region and over this time period may be a limiting factor. Variations in mixed layer temperature due to surface heat flux anomalies may also contribute to the interdecadal sea level signal at Hawaii.
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Zhang, Ran, Jiabei Fang, and Xiu-Qun Yang. "What Kinds of Atmospheric Anomalies Drive Wintertime North Pacific Basin-Scale Subtropical Oceanic Front Intensity Variation?" Journal of Climate 33, no. 16 (2020): 7011–26. http://dx.doi.org/10.1175/jcli-d-19-0973.1.
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ABSTRACTThe basin-scale subtropical oceanic front zone (STFZ) is a key region for midlatitude air–sea interaction in the North Pacific. However, previous studies considered midlatitude sea surface temperature (SST) variabilities as a response to atmospheric stochastic forcing. With reanalysis and observational data, this study investigates what kinds of atmospheric anomalies drive the wintertime North Pacific STFZ intensity variation. Lead correlations show that prior to the STFZ’s enhancement, there exist persistent atmospheric anomalies characterized by a negative-phase Arctic Oscillation (AO) and a positive-phase Pacific–North American (PNA) pattern, lasting for up to 80 and 50 days and peaking at 20- and 8-day leads, respectively. It is further found that the long-lasting negative-phase AO is conducive to stronger low-tropospheric baroclinicity at around 40°N over North Pacific where there is a climatological baroclinic region. The stronger baroclinicity leads to more synoptic transient eddy activities, promoting an equivalent barotropic low geopotential height anomaly north of STFZ via transient eddy vorticity forcing. The geopotential height anomaly propagates downstream, triggering a PNA-like pattern. With such an AO-promoted atmospheric internal wave–flow feedback, the regional PNA pattern is intensified and embedded in the annular AO mode, accompanied with an intensified Aleutian low and surface westerly wind that peak at an 8-day lead, preconditioning a persistent (nonstochastic) atmospheric forcing on the STFZ. The intensified surface westerly predominantly tends to drive a southward Ekman transport and increase upward surface turbulent heat fluxes into the atmosphere through increasing surface wind speed and sea–air temperature difference, amplifying the underlying negative SST anomaly and cross-frontal meridional SST gradient, ultimately intensifying the STFZ.
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Zhang, Yu, Shang-Ping Xie, Yu Kosaka, and Jun-Chao Yang. "Pacific Decadal Oscillation: Tropical Pacific Forcing versus Internal Variability." Journal of Climate 31, no. 20 (2018): 8265–79. http://dx.doi.org/10.1175/jcli-d-18-0164.1.
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The Pacific decadal oscillation (PDO) is the leading mode of sea surface temperature (SST) variability over the North Pacific (north of 20°N). Its South Pacific counterpart (south of 20°S) is the South Pacific decadal oscillation (SPDO). The effects of tropical eastern Pacific (TEP) SST forcing and internal atmospheric variability are investigated for both the PDO and SPDO using a 10-member ensemble tropical Pacific pacemaker experiment. Each member is forced by the historical radiative forcing and observed SST anomalies in the TEP region. Outside the TEP region, the ocean and atmosphere are fully coupled and freely evolve. The TEP-forced PDO (54% variance) and SPDO (46% variance) are correlated in time and exhibit a symmetric structure about the equator, driven by the Pacific–North American (PNA) and Pacific–South American teleconnections, respectively. The internal PDO resembles the TEP-forced component but is related to internal Aleutian low (AL) variability associated with the Northern Hemisphere annular mode and PNA pattern. The internal variability is locally enhanced by barotropic energy conversion in the westerly jet exit region around the Aleutians. By contrast, barotropic energy conversion is weak associated with the internal SPDO, resulting in weak geographical preference of sea level pressure variability. Therefore, the internal SPDO differs from the TEP-forced component, featuring SST anomalies along ~60°S in association with the Southern Hemisphere annular mode. The limitations on isolating the internal component from observations are discussed. Specifically, internal PDO variability appears to contribute significantly to the North Pacific regime shift in the 1940s.
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Malloy, Kelsey M., and Ben P. Kirtman. "Predictability of Midsummer Great Plains Low-Level Jet and Associated Precipitation." Weather and Forecasting 35, no. 1 (2020): 215–35. http://dx.doi.org/10.1175/waf-d-19-0103.1.
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Abstract Warm-season precipitation in the U.S. “Corn Belt,” the Great Plains, and the Midwest greatly influences agricultural production and is subject to high interannual and intraseasonal variability. Unfortunately, current seasonal and subseasonal forecasts for summer precipitation have relatively low skill. Therefore, there are ongoing efforts to understand hydroclimate variability targeted at improving predictions, particularly through its primary transporter of moisture: the Great Plains low-level jet (LLJ). This study uses the Community Climate System Model, version 4 (CCSM4), July forecasts, made as part of the North American Multi-Model Ensemble (NMME), to assess skill in reproducing the monthly Great Plains LLJ and associated precipitation. Generally, the CCSM4 forecasts capture the climatological jet but have problems representing the observed variability beyond two weeks. In addition, there are predictors associated with the large-scale variability identified through linear regression analysis, shifts in kernel density estimators, and case study analysis that suggest potential for improving confidence in forecasts. In this study, a strengthened Caribbean LLJ, negative Pacific–North American (PNA) teleconnection, El Niño, and a negative Atlantic multidecadal oscillation each have a relatively strong and consistent relationship with a strengthened Great Plains LLJ. The circulation predictors, the Caribbean LLJ and PNA, present the greatest “forecast of opportunity” for considering and assigning confidence in monthly forecasts.